“Wafer bumping” refers to a semiconductor packaging technology that prepares microchips for flip chip attachment onto an integrated circuit package or other electronic assembly. There are numerous different methods of wafer bumping, but the process typically involves placing tiny amounts of electrically conductive material onto the input/output pads of a wafer and then subjecting the wafer assembly with the electrically conductive material to a thermal process known as mass solder reflow. Controlling the wafer temperature is critically important to allow the solder to reflow while at the same time preventing damage to the wafer.
One prior art method of performing a solder reflow process is known in the industry as hot-plate technology. Hot-plate technology utilizes conduction, where the wafer is placed on a flat thermal mass precisely maintained at a given temperature. Conduction heating of the type employed in hot-plate technology is beneficial because the temperature of the wafer will not go higher than the temperature of the thermal mass (hot-plate) on which the wafer is sitting. Traditionally, hot-plate technology requires a system having multiple isolated hot plate stages and a means of moving the wafer to be reflowed from stage to stage to achieve proper heating.
There are several disadvantages to the prior art hot-plate type reflow system. First, because of their thermal mass, hot plates are slow to heat up and to cool down. Therefore, a lot of production time is lost when it is necessary to change set point temperature for a high temperature solder alloy to a lower temperature for a low temperature solder. Second, a pusher bar is typically used to push wafers from stage to stage. Spacing between pusher bars are established for a particular sized wafer, for example, eight-inch and twelve-inch wafers, etc. If the equipment is used with smaller wafers than it was designed for, production capacity is lost. Third, fluxes residues are formed from the wafers as they are heated, and the fluxes residues accumulate on plate surfaces. One source of flux is an organic acid that is put on a wafer to provide viscosity and to remove oxides on metal surface that inhibit good electrical connections. When passing through the various hot plates, the wafers get dirty from being pushed through the condensed fluxes, and have to be cleaned in an additional, costly step.
Fourth, commercially available hot plate equipment does not provide a controlled atmosphere surrounding the reflow and cooling areas that is capable of maintaining an inert atmosphere or reactive gas atmosphere that contains less than five parts per million oxygen molecules, and so allowing oxidation to occur. Fifth, the gas that is used in the prior art systems to control atmosphere causes wafer movement which makes in line processing using cassette to cassette loading equipment more difficult to use. Sixth, the condensed fluxes must frequently be manually removed from numerous parts of the hot-plate equipment causing significant lost production time and considerable expense.